The present invention relates generally to gas turbine engines, and, more specifically, to borescope plugs therein.
In a bypass turbofan gas turbine engine, a core engine is spaced radially inwardly from an outer casing to define a bypass duct therebetween. The core engine includes its own casing, the inner casing, and powers a fan which bypasses a portion of pressurized air therefrom through the bypass duct and over the inner casing surrounding the core engine.
The core engine includes a multi-stage axial flow compressor which further pressurizes the air received from the fan, which is then discharged through a diffuser into a combustor wherein it is mixed with fuel and ignited for generating hot combustion gases which flow downstream through corresponding high and low pressure turbines, which in turn power the compressor and fan, respectively.
In order to periodically inspect the condition of the internal parts of the engine, such as the combustor and compressor and turbine blades and vanes, borescope ports are provided in the engine casings and frames. Through these ports optical borescope instruments can be inserted into the core engine to enable visual observation of the condition of internal engine parts from outside the engine, without having to disassemble the engine.
Borescope plugs are used to close and seal these ports to prevent high pressure, high temperature engine gases from escaping therethrough during engine operation. The borescope plugs are threaded into tapped bosses in the core engine casings. The plugs are torqued tightly into the bosses, and the resultant preload and friction force prevents the plug from unthreading, and becoming loose. Thus, the borescope plug is fixed to the core engine casing.
In general, as the engine approaches operating temperature and pressure, the core engine casings expand more than the outer casing in both radial and axial directions due to the differences in temperatures and materials. In addition, the vibration amplitude and frequency response of the core engine casings and the outer casing will be different. For these reasons, the borescope plug must be free to move axially, radially, and circumferentially, relative to the outer engine casing.
To prevent excessive leakage of the bypass duct air through the outer casing, a floating seal is used around the borescope plug where it penetrates the outer casing. In a typical embodiment, the seal is a flat washer which has a close fit with the borescope plug. This washer is secured to the outside of the outer casing by means of a retainer plate which has a recess to contain the floating seal washer, yet allows it to move freely in axial and circumferential directions.
If the borescope plug is not properly torqued during assembly, it may subsequently loosen and be ejected from the engine during operation. In the case of a compressor borescope plug, the compressed air is relatively hot and is expelled as a jet through the borescope port, and not only adversely affects engine performance, but may cause heat damage to impacted components. For example, the outer casing may be formed of a conventional composite material having a lower operating temperature than that of metal casings. Impinging hot leakage flow may therefore degrade the composite material of the outer casing causing excessive damage thereto requiring complete replacement thereof.
Accordingly, it is desired to provide a self-retained borescope plug for accommodating inadvertent disconnection of the plug from its mounting boss.